The present invention relates to an apparatus and method for activating electrically programmable fusible links in VLSI circuits, specifically fusible links having a reduced interlink spacing, without damaging the underlying substrate.
High density dynamic random access memories (DRAMS) are designed with memory cell redundancy. The redundant memory cells avoid the loss of an entire memory in the event that a minor number of memory cells are not functioning. Activation of the redundant memory cells is accomplished by fusible links which are strategically placed throughout the memory. Activation of a fusible link results in the disabling of the defective memory cell, while enabling in its place a redundant memory cell.
The process of "blowing" fusible links is implemented by heating the fusible link which is to be blown. The heated fusible link melts or evaporates, creating an open circuit for replacing the defective memory cells with a functional cell.
The fusible links are made of aluminum, copper and other high conductive metal or metal alloys. The conductive fusible link generally has a central width portion which is smaller than the ends, to reduce the amount of energy necessary to melt the fusible link to create an open circuit condition.
The melting of fusible links may be accomplished using a laser beam having a controlled beam width. Additionally, the fusible links may be opened by applying a high current thereto, heating the fusible links as a result of power dissipation in the fusible link which is sufficient to melt the fusible links.
Utilizing the laser for blowing the fusible links requires that two conditions be observed to avoid damage to the circuit. The first is to have a beam width which is sufficiently narrow to blow only a single fusible link without inadvertently blowing an adjacent fusible link. The second condition which must be observed is avoiding damage to the underlying silicon substrate which supports the fuse elements.
These requirements are at odds with each other in that while it is desirable to decrease the distance between fusible links, i.e., pitch, there is a corresponding reduction in the laser beam width which produces a high energy beam which may damage the silicon substrate. In order to further increase the density of DRAMS, it is therefore desirable to reduce the pitch between fusible links, while at the same time being able to blow the fusible links without damaging the underlying silicon surface or adjacent fusible links.